1. Field of the Invention
The present invention is a process for the in situ bioremediation of chromium (VI) (Cr(VI))-bearing solids, including soils, sediments and wastes. In particular, the present invention relates to a process for the in situ treatment of Cr(VI)-bearing solids wherein Cr(VI) is bioremediated to Cr(III) without the need for removal of the solids from their resting place.
2. Description of the Related Art
Much effort is being expended on remediation of waste disposal sites contaminated with Cr(VI) in many localities across the country. These Cr(VI)-bearing solids potentially pose a health threat as compared with the low toxicity of Cr(III)-bearing solids.
One type of Cr(VI)-bearing waste is the residue produced by a typical chromite ore roasting process, in which a portion of the Cr(III) in chromite ore is oxidized to Cr(VI) by roasting in a kiln and then water-soluble Cr(VI) salts are extracted from the roasted ore. The chromite ore processing residue (COPR) contains Cr(VI), due to incomplete leaching, and is usually highly alkaline, due to the use of lime (CaO) in the roasting process.
It is typical for Cr(VI) to be present in COPR at concentrations ranging from 10,000 to 20,000 mg/kg, and the Cr(VI) fraction of the total Cr is generally in the range from 1% to 13%. However, when COPR has been mixed with other materials, it is common that the Cr(VI) concentration in the mixture, as a fraction of the total chromium concentration, varies widely.
Cr(VI) salts are very soluble in water, in comparison to Cr(III), which precipitates as a hydroxide at neutral and alkaline pH. Cr(VI) is actually present as a negative ion (anion) in water as opposed to Cr(III) which is a positive ion (cation). Anions are usually more highly mobile in soils than cations, which are exchanged with other cations in soils. The result is that Cr(VI) tends to be quite soluble and mobile and Cr(III) tends to be relatively insoluble and immobile. Conversion of Cr(VI) to Cr(III) has the benefit of greatly reducing the migration potential of the chromium in the environment.
Biological reduction of Cr(VI) to Cr(III) has been demonstrated in the laboratory and in the field. In a report of research to The Engineering Foundation & American Society of Civil Engineers in 1979, Higgins reported on the biological reduction of Cr(VI) to Cr(III) and subsequent removal from a wastewater stream. The researcher utilized laboratory soil columns to investigate the movement of heavy metals to groundwater when treated wastewater containing Cr(VI) and Cd was applied to agricultural soils for irrigation. The investigator found that initially the Cr(VI) percolated freely through the soil columns, but that with time, the Cr(VI) concentrations in the percolate decreased. Significant bacterial growth was noted on the surface of the columns. Chromium removal was postulated to be due to the biological reduction of Cr(VI) to Cr(III) followed by either precipitation of the hydroxide or adsorption or both. Food for bacterial growth was supplied by the residual biological oxygen demand (BOD) in the percolating water.
U.S. Pat. No. 5,155,042 to Lupton et at., relates to the bioremediation of Cr(VI)-bearing solids, specifically COPR. In the process, Cr(VI) is leached from the solids by injecting an acidic solution into the solids at one location and removing the leachate from a second location for treatment in an external biological reactor to which is added sulfate-reducing anaerobic bacteria, sulfates and other nutrients, as needed for the growth of the bacteria. In the reactor, Cr(VI) is biologically reduced to Cr(III) which is then precipitated as a hydroxide and removed from solution using solids separation processes. Acid is then added to the sulfate-reducing anaerobic bacteria-laden solution to maintain a pH of 6.5 to 9.5 and the solution is recirculated into the Cr(VI)-bearing solids to promote in situ reduction and leaching of the remaining Cr(VI). It was noted that COPR exhibited an "alkaline rebound" effect where after the addition of sufficient acid to reduce the pH to the range of 6.5 to 9.5, the pH slowly rose to above 9.5, due to the slow release of alkalinity from the COPR. They noted that the soluble Cr(VI) concentration in the COPR must be less than 200 mg/l and the pH stabilized before a self-sustaining population of sulfate-reducing anaerobic bacteria could be maintained in situ. They therefore proposed a process in which multiple applications of acid and sulfate-reducing anaerobic bacteria are necessary, and in which external treatment of the leachate in a biological reactor is used to reduce the Cr(VI) to Cr(III).
U.S. Pat. No. 5,285,000 to Schwitzgebel is directed to the in situ chemical treatment of Cr(VI) contaminated soil. The method first uses a ferrous iron containing solution to reduce Cr(VI) to Cr(III) and coprecipitate the resulting Fe(OH).sub.3 and Cr(OH).sub.3 with other heavy metals. A sodium silicate gel-forming solution is added to reduce leaching of the metals.
U.S. Pat. No. 5,304,710 to Kigel et al. relates to an ex situ process for chemically treating chromium ore waste-contaminated soils by acidification, chemical reduction, neutralization and stabilization. The method includes the steps of soil grinding, acidification to a pH less than or equal to 3, reduction of Cr(VI) to Cr(III) using a ferrous iron salt, raising the pH with an alkaline agent such as lime, precipitating the chromium and iron as hydroxides, and if needed to improve physical strength, stabilizing the mixture by adding cement, cement kiln dust, fly ash, slag or other agents.
U.S. Pat. No. 5,202,033 to Stanforth et al. is directed to the in situ chemical treatment of Cr(VI) contaminated soil. The method of treating solid waste in soil or solid waste containing unacceptable concentrations of chromium includes mixing the waste or soil in situ with ferrous sulfate. The method consists of adding ferrous sulfate and a pH controlling agent such as magnesium oxide, magnesium hydroxide, calcium oxide or calcium hydroxide, to the soil or waste, and mixing under conditions which support reactions that will convert the chromium to a non-leachable form. The treatment additives can be introduced and contacted with the soil or waste by the following techniques: spreading the additives on top of the soil or waste and mixing with a mechanical device, such as a rotary tiller; adding the treatment chemical through an infiltration gallery as a solution or slurry; injecting a soluble additive through injection nozzles or injection wells; and, adding a treatment additive through a hollow-shaft auger and mechanical mixing.
Methods which involve treatment of Cr(VI)-bearing solids by percolating acid through the media suffer from two deficiencies: the acids react with the solids to significantly reduce the hydraulic permeability of the media therefore limiting the ability to continue to percolate treatment materials; and, the acid reacts with the first solid material it comes in contact with, producing a lower than desirable pH in the pore water near the point of injection and higher than desirable pH away from the point of injection. The result would be a significant variation in pH of the media, with little of the media at the desired pH range of 6.5 to 9.5.
Likewise, percolation is an inefficient method of distributing bacteria into a Cr(VI)-bearing solids media. The tendency will be for the bacteria to be removed by filtration in the media. Bacteria that are not filtered out would tend to be single cells (to avoid the filtration effects of the media) and would be exposed to high concentrations of Cr(VI), reducing the number of viable bacteria that could be injected through percolation.
U.S. Pat. No. 5,155,042 to Lupton et at., is limited to the use of sulfate reducing bacteria for Cr(VI) reduction. One of the patent holders, Defilipi, in "Bioremediation of Hexavalent Chromium in Water, Soil and Slag Using Sulfate Reducing Bacteria", a preprint from Handbook of Process Engineering for Pollution Control and Waste Minimization, ed. by D. L. Wise and D. J. Tratolo, determined that sulfate reducing bacteria produce H.sub.2 S, which then reacts with Cr(VI) to reduce it to Cr(III), which then precipitates as chromic hydroxide. One potential problem with this process is the generation of H.sub.2 S, a toxic gas.
Other researchers have demonstrated that bacteria other than those that are sulfate reducing are effective at reducing Cr(VI) to Cr(III). Higgins demonstrated that the bacteria present in the effluent from a domestic wastewater treatment plant would reduce Cr(VI).
Blake et al in "Chemical Transformation of Toxic Metals by a Pseudomonas Strain from a Toxic Waste Site", Environmental Geochemistry and Health, Vol. 15, No. 2, 1993, studied the bacterium Pseudomonas maltophilia. Their tests demonstrated that "the reduction of Cr(VI) was catalyzed by a membrane-bound chromate reductase". Ohtake and Hardyo's tests of Enterobacter cloacae found that the bacterium anaerobically reduced Cr(VI) at the cell surface. The reduced chromium then precipitated as an insoluble metal hydroxide. Their tests also indicated that the most favorable pH for the reaction was 7. See, "New Biological Method for Detoxification and Removal of Hexavalent Chromium", Water Science and Technology, Vol. 25, No. 15, 1992.
Accordingly, it is an object of the present invention to provide an effective and efficient method for the in situ biological reduction of Cr(VI)-bearing solids by in-place mechanical mixing with organic nutrients, naturally occurring bacteria, and mineral acid or bases without excavation of the solids.
These and other objects of the present invention will become apparent upon review of the following specification and the claims appended thereto.